1. Technical Field
The invention is related to position tracking systems generally and particularly to tracking pilot helmet position in the cockpit of a flight vehicle or simulator using ultrasonic transmitters mounted in the cockpit and ultrasonic transducers mounted on the helmet, or vice versa.
2. Background Art
Position tracking systems for monitoring the position of an article such as a pilot's helmet in a cockpit are useful for such purposes as maintaining a computer-generated projected image display in the pilot's field of view, for example. Other purposes include tracking any part of a person's body such as the head, hand or finger, so that the person may generate inputs to a computer by the movement of his head, hand or finger, for the control of cursor movement in a computer display, for example. Other applications include robotics and weapon systems.
The advantages of employing ultrasonic waves to perform such position tracking over techniques employing electromagnetic signals (such as those disclosed in U.S. Pat. No. 4,742,356 to Kuipers and U.S. Pat. No. 4,303,394 to Berke et al.) are well-known. Essentially, electromagnetic systems are far more vulnerable to interference from many noise sources. Ultrasonic position tracking techniques are well-known. For example, U.S. Pat. No. 4,807,202 to Cherri et al. discloses an ultrasonic tracking system in which the orientation and position of a movable object (such as a helmet) in a closed frame of reference (such as a cockpit) is continually tracked by following the six degrees of freedom of movement of the moveable object. This is accomplished, as illustrated in FIG. 1 hereof, by mounting three ultrasonic transmitters 100, 102, 104 in three different locations in the closed frame of reference 106 and mounting three ultrasonic transducers 108, 110, 112 in three different locations on the moveable object 114. Each one of the three transmitters 100, 102, 104 transmits an ultrasonic acoustic wave signal at a different ultrasonic frequency, all of which are received at each one of the three transducers 108, 110, 112. The three ultrasonic frequencies received at each transducer are separated into three received signals, so that a total of nine signals are received and processed. A tracking processor 116 processes each of the nine signals to provide the distance between the transducer and the transmitter corresponding to the frequency of the received signal, thus providing nine distances. Using well-understood principles, the tracking processor 116 computes the instantaneous position and orientation of the moveable object 114 with respect to the closed frame of reference from the nine distances. This computation uses the locations of the transmitters in the closed frame of reference and the locations of the transducers with respect to the frame of reference of the moveable object.
Various methods of transmitting and processing the ultrasonic signals are employed, all with varying degrees of limited performance. The basic limitation of such methods is that they are slow, limiting the rate at which the position and orientation of the movable object can be repeatedly computed. Most of the methods employ a pulsed ranging ultrasonic technique, such as the techniques disclosed in U.S. Pat. No. 4,853,863 to Cohen et al., U.S. Pat. No. 4,807,202 to Cherri et al., U.S. Pat. No. 3,836,953 to Rotier and U.S. Pat. No. 3,777,305 to Stoutmeyer.
The above-referenced patent to Cohen et al. discloses an ultrasonic position tracking technique in which the frequency shift of the ultrasonic signal due to helmet movement is measured and integrated to provide a displacement value from which the transmitter-to-receiver range is computed.
The problem with the pulse or sequential techniques is that the time required for the receiver to acquire a sufficient ultrasonic signal from which the transmitter-to-receiver range can be inferred limits the rate at which the helmet position can be tracked. For example, the ultrasonic pulsed ranging techniques are limited by the time of flight between the transmitter and the receiver. Generally, it is believed that such sequential techniques produce an updated position measurement for a given sensor at a rate not exceeding on the order of 10 Hz. Most users would be pleased by a 100 Hz update rate. Pulse or sequential techniques are limited to a total time of 9 time-of-flight intervals, or about 30 ms. In a room with echoes, this figure may be 3 or 4 times as large. Consequently, the maximum update rate for sequential techniques is 30 Hz, although a more practical update rate is 10 Hz. Another problem with sequential techniques is that the data is incoherent. The 9 ranges are not simultaneous. As will be shown below, the present invention provides the 9 ranges simultaneously as coherent data, a significant advantage over the prior art.
The problem with the technique of integrating the frequency shift is that its maximum update rate is limited by the time required to measure frequency, compute the doppler shift and then integrate it so as to produce a displacement value from which to compute a range.
What is needed is an ultrasonic position tracker which will track the moveable object position at far higher rates and provide the nine measurements coherently or simultaneously. However, given the present state of the art, such tracking rates do not seem possible.